Polyimides for STN displays

ABSTRACT

Polyamide esters which can be prepared from tetracarboxylic dianhydrides and can be converted into polyimides by alcohol cleavage with cyclization, characterized in that the radicals OR of the ester groups --COOR which are substituted during the polyimide formation are alkoxy radicals having 2 to 5 C atoms which are monosubstituted or polysubstituted by fluorine. 
     Polyimides prepared therefrom are suitable as orientation layers in liquid-crystal display elements and in optical wave guides.

BACKGROUND OF THE INVENTION

The invention relates to polyamide esters which can be prepared fromtetracarboxylic dianhydrides and can be converted into polyimides byalcohol cleavage with cyclization, characterized in that the radicals ORin the ester groups --COOR, which are substituted during the polyimideformation, are alkoxy radicals having 2 to 5 C atoms which aremonosubstituted or polysubstituted by fluorine.

Polyimides are known as orientation layers in liquid-crystal displaysand have proven successful due to their good optical and thermalproperties. They are also employed, in particular, in supertwistliquid-crystal display elements (SLC display elements). This is taken tomean relatively highly twisted display elements with a twist anglehaving a value in between 160° and 360°, such as, for example, thedisplay elements according to Waters et al. (C. M. Waters et al., Proc.Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe,Japan), STN-LCDs (German Offenlegungsschrift 3,503,259), SBE-LCDs (T. J.Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M.Schadt and F. Lennhouts, Appl. Phys. Lett. 50 (198U), 236), DST-LCDs(European Published Specification 0,246,842) or BW-STN-LCDs (K. Kawasakiet al., SID 87 Digest 391 (20.6)).

To produce supertwist display elements, orientation materials arerequired which induce a high tilt angle of the liquid crystal in thedisplay; the angle should be at least about 3°.

It is known that polyimides prepared, for example, by reaction of thefollowing components ##STR1## cause tilt angles of greater than 3° (forexample European Published Specification 231,781).

Polyimides of this type are usually prepared by first reactingtetracarboxylic acids with diamines to give polyamidocarboxylic acids,which are then cyclized with elimination of water to form polyimides(for example European Published Specification 0,249,881, EuropeanPublished Specification 0,231,781 and European Published Specification0,219,336). Alternatively, the tetracarboxylic anhydride can first beconverted into another derivative, for example a dicarboxylic aciddiester, after which the ester component is further reacted with anamine component to form the polyamidocarboxylic acid (U.S. Pat. No.4,467,000).

European Published Specification 0,209,114 describes polyamide esterswhich can be converted into polyimides by alcohol cleavage. However, theinvention relates to amphiphilic compounds, i.e. the alcohol componentcontains long alkyl chains.

Polyamide esters have furthermore been described in connection withphotopolymerizable polyamide acid derivatives (for example EuropeanPublished Specification 0,203,372).

SUMMARY OF THE INVENTION

The invention had the object of developing further polyimide precursorswhich can easily be converted into polyimides, the polyimides and theirprecursors having further advantages over the prior art.

This object is achieved by the provision of the polyamide estersaccording to the invention.

Surprisingly, it has been found that polyamide esters whose alcoholcomponent contains alkoxy radicals which are monosubstituted orpolysubstituted by fluorine cyclize to form polyimides even at very lowtemperatures. The imidation begins even at temperatures well below 200°C. This corresponds approximately to the lower range of known imidationtemperatures. However, it is much more surprising that the polyamideesters according to the invention are virtually to completely colourlessand also do not discolour during the imidation. The known polyamideesters are usually already yellow, and some are even yellow-brown oreven grey-green (for example EP 0,209,114).

This opens up a further broad field of application for the polyimidesaccording to the invention. Due to their advantageous opticalproperties, they can be employed in optical wave guides and thus in avery wide variety of optical components.

The invention therefore relates to polyamide esters which can beprepared from tetracarboxylic dianhydrides and can be converted intopolyimides by alcohol cleavage with cyclization, characterized in thatthe radicals OR of the ester groups --COOR which are substituted duringthe polyimide formation are alkoxy radicals having 2 to 5 C atoms whichare monosubstituted or polysubstituted by fluorine.

The invention furthermore relates to a process for the preparation ofthe polyamide esters according to the invention which is characterizedin that a tetracarboxylic dianhydride is reacted with an aliphaticalcohol having 2 to 5 C atoms which is monosubstituted orpolysubstituted by fluorine, and with a diamine in any desired sequence,--COO(H) groups formed after the first reaction step optionally beingconverted into reactive derivatives.

The invention also relates to polyimides prepared from the polyamideesters according to the invention, and to liquid-crystal displayelements and optical components containing the polyimides according tothe invention.

The polyamide esters according to the invention are compounds containingstructural units of the general formula I ##STR2## A is a tetravalentorganic radical having at least two C atoms, B is a divalent organicradical having at least two C atoms, and R is an alkyl radical havingtwo to five C atoms which is monosubstituted or polysubstituted byfluorine.

The components from which the compounds of the formula I aresynthesized, independently of the synthetic route, are firstly atetracarboxylic dianhydride (contains A), secondly a diamine (containsB), and thirdly an alcohol (contains R).

Examples of tetracarboxylic dianhydrides are aromatic dianhydrides ofpyromellitic acid, benzophenonetetracarboxylic acid,biphenyltetracarboxylic acid, and alicyclic dianhydrides ofcyclobutane-, cyclopentane- and cyclohexanetetracarboxylic acid. For thepreparation of the polyamide esters according to the invention, thetetracarboxylic dianhydrides can be employed alone or mixed with oneanother.

Examples of diamines are aromatic diamines, such as p-phenylenediamine,m-phenylenediamine, o-phenylenediamine, 4,4'-diaminodiphenylmethane,4,4'-diaminodiphenyl ether, 2,2-bis(4-aminophenyl)propane,diaminodiphenyl sulfone, diaminobenzophenone, 1,5-diaminonaphthalene,1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene,4,4'-di(4-aminophenoxy)diphenyl sulfone,2,2-bis[4-(4-aminophenoxyphenyl)propane],2,2-bis(4-aminophenyl)hexafluoropropane,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, and alicyclicdiamines, such as, for example, 1,4-diaminocyclohexane,1,3-diaminocyclohexane, 4,4'-diaminodicyclohexylmethane,4,4'-diamino-3,3'-dimethyldicyclohexylmethane, 4,4'-diaminodicyclohexylsulfone, 4,4'-diaminodicyclohexyl ether, and aliphatic diamines, such as1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane,1,5-diaminopentane, 1,6-diaminohexane, 1,7-diaminoheptane and1,8-diaminooctane.

For the preparation of the polyamide esters according to the invention,the diamines can be employed alone or mixed with other diamines.

It is also possible to employ other tetracarboxylic dianhydrides anddiamines, for example those described or listed in DE 3,107,519, EP0,209,114, EP 0,219,336, EP 0,231,781 and EP 0,249,881.

The third component for the synthesis of the polyamide esters accordingto the invention, the alcohol of the formula I containing the radical R,is an alcohol having 2, 3, 4 or 5 C atoms which is at leastmonosubstituted by fluorine, for example 2,2,2-trifluoroethanol. Thepolyamide esters according to the invention can be prepared by reactinga tetracarboxylic dianhydride with an alcohol to form a diester or witha diamine to form a corresponding oligomeric or polymeric amidic acid,and converting the diester or amidic acid into the polyamide ester usingthe final component, the diamine or the alcohol, either directly or viaa reactive derivative of the free COO(H) group.

The individual reactions are common and customary carbonyl reactionsknown to those skilled in the art and are described in the relevantstandard works of preparative organic chemistry, such as, for example,in Houben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg Thieme Verlag, Stuttgart.

For example, the tetracarboxylic dianhydride can first be converted intothe diester in a suitable solvent, such as, for example,tetrahydrofuran. The resultant COO(H) groups can subsequently beconverted into more reactive groups, preferably under mild reactionconditions. They are usually converted into carboxylic acid halides,preferably into carboxylic acid chlorides, using thionyl chloride. Thereaction to form the polyamide ester is likewise preferably carried outunder mild conditions, using the desired diamine with addition of anauxiliary base, for example pyridine.

Alternatively, the tetracarboxylic acid diesters can also bepolycondensed directly with the diamine to form the polyamide ester; thecondensing agent employed is preferably a carbodiimide, for exampledicyclohexylcarbodiimide (DCC), as described in DE 3,411,660.

The polyamide esters according to the invention are distinguished by thefact that polyamide formation sets in at very low temperatures, wellbelow 200° C.

At the same time, the polyamide esters are themselves virtually tocompletely colourless and also do not discolour during the imidation.

The polyimides prepared by the process according to the invention arethus distinguished by their virtual or complete lack of colour comparedwith those prepared by known processes, although they are in some casesstructurally identical therewith. The polyimides according to theinvention are predominantly imidated, i.e. they contain only a lowproportion of ester groups, if any at all. The degree of imidation isgenerally 95% or more.

The polyimides according to the invention can be employed, on the onehand, as orientation layers in liquid-crystal display elements, inparticular also in those having high twist angles, such as STN, SBE oralso OMI displays, in which high tilt angles of the liquid crystal arenecessary.

For the production of polyimide orientation layers, the polyamide estersaccording to the invention are applied to a substrate (for example glassor metal) by known methods, for example by spin coating, spraying,brushing, printing or simple dipping.

It is expedient here to use a polymer solution in a suitable solvent,such as dimethylformamide, dimethylacetamide, dimethyl sulphoxide,N-methyl-2-pyrrolidone or in a comparable solvent. After drying, curingis subsequently carried out in vacuo. The imidation begins well below200° C., in individual cases even below 150° C. Heating is carried outfor one hour at about 200°-250° C.

The polyimides according to the invention are furthermore particularlysuitable for use in optical wave guides due to their advantageousoptical properties. This opens up the way to optical components, forexample in the area of integrated optics or sensor and communicationtechnology. Optical components are described, for example, in EP0,218,938.

The working examples below are intended to illustrate the invention,without representing a limitation.

EXAMPLE 1 Preparation of Polyamide Esters

14.39 ml (0.2 mol) of 2,2,2-trifluoroethanol are added dropwise withstirring at 15° C. to a solution of 32.2 g (0.1 mol) of3,3',4,4'-benzophenonetetracarboxylic dianhydride in 234 ml oftetrahydrofuran. After the mixture has been stirred for 30 minutes,33.42 ml of pyridine are added dropwise at 15° C. After a further 3hours, the mixture is cooled to -10° C., and 14.55 ml of thionylchloride are slowly added dropwise with vigorous stirring. After 2 hoursat -10° C., a solution of 46.68 g (0.09 mol) of2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 66.8 ml ofdimethylacetamide is added dropwise, and the mixture is stirred for afurther 30 minutes. After the mixture has been warmed to 15° C., 15.02ml of acetyl chloride are added and, after a further 4 hours, 55.54 mlof ethanol are added. The solution is transferred into water containingTitriplex®-IV (Messrs. Merck, 5 l of H₂ O/250 mg of Titriplex®-IV). Theresultant precipitate is filtered off with suction, dried at roomtemperature and dissolved in tetrahydrofuran, and the solution isstirred every 2 hours with the ion exchanger III and I (Messrs. Merck).After the second ion exchanger has been filtered off with suction, themixture is stirred into H₂ O/Titriplex®-IV (5 l/spatula tip) and thesolid is filtered off with suction and dried as above. A virtuallycolourless (only slightly yellowish) powder of the polyamide ester a isobtained.

The polyamide esters are prepared analogously using the components

    ______________________________________                                         ##STR3##                                                                     A                B                                                            ______________________________________                                        b)                                                                                    ##STR4##                                                                                    ##STR5##                                                c)                                                                                    ##STR6##                                                                                    ##STR7##                                                d)                                                                                    ##STR8##                                                                                    ##STR9##                                                e)                                                                                    ##STR10##                                                                                   ##STR11##                                               f)                                                                                    ##STR12##                                                                                   ##STR13##                                               g)                                                                                    ##STR14##                                                                                   ##STR15##                                           

Polyimides

The polymers 1 a to 1 g are spin-coated from a 3% solution inN-methyl-2-pyrrolidone (NMP) onto glass plates (Baltracon Z 20) coatedwith ITO (indium/tin oxide), and subsequently cured for one hour at 250°C. in vacuo. The polyimide-coated glass plates are subsequently rubbedin one direction with a rayon cloth (M. E. Becker et al., Mol. Cryst.Liq. Cryst. 132, 167-180 (1986)). In order to determine the tilt angle,a cell with a distance between the glass plates of 50 μm and anantiparallel arrangement (with regard to the rubbing direction) of theglass plates to one another is constructed. The cell is filled bycapillary forces by dipping it into the liquid crystal. The liquidcrystal is subsequently isotropized. The polyimides prepared frompolyamide esters 1 d and 1 g exhibit a tilt angle of 3.7° as orientationlayers on investigation with the liquid crystal mixture ZLI-2293,Messrs. Merck. The tilt angle is determined as described by A. Mosleyand B. Nicholas in Displays, 1987, pp. 17-21.

EXAMPLE 3

The polyamide ester 1 d is dissolved to form a 20% solution in NMP and,as described in Example 2, spin-coated onto a glass plate (thickness:1.1 mm) and cured. A transparent, colourless film whose thickness wasdetermined as being 1 μm using an α-stepper is obtained.

The absorption of the film is determined using a spectrometer. The filmexhibits an absorption of less than 0.01 at 800 nm and less than 0.015at 400 nm.

For comparison, a typical, commercially available, yellowish film(Problmide 32, Ciba-Geigy) was measured. The film exhibits absorptionproperties which are typical of such films: 0.03 at 800 nm and 1.4 at400 nm.

We claim:
 1. Polyamide esters which can be prepared from tetracarboxylicdianhydrides and can be converted into polyimides by alcohol cleavagewith cyclization at a curing temperature of less than 250° C.,characterized in that the radicals OR of the ester groups --COOR whichare substituted during the polyimide formation are alkoxy radicalshaving 2 to 5 C atoms which are monosubstituted or polysubstituted byfluorine.
 2. Process for the preparation of polyamide esters accordingto claim 1, characterized in that a tetracarboxylic dianhydride isreacted with an aliphatic alcohol having 2 to 5 C atoms which ismonosubstituted or polysubstituted by fluorine, and with a diamine, inany desired sequence.
 3. Process according to claim 2, characterized inthat the first reaction is with an alcohol.
 4. Polyimides which havebeen prepared from polyamide esters, wherein said polyamide esters a)can be prepared from tetracarboxylic dianhydrides, b) can be convertedinto polyimides by alcohol cleavage with cyclization, and c) have alkoxyradicals OR of the ester groups --COOR which are substituted during thepolyimide formation, have 2 to 5 C atoms, and are monosubstituted orpolysubstituted by fluorine.
 5. A method of using polyimides accordingto claim 4 as orientation layers in liquid-crystal display elementswhich comprise incorporating said polyimide into an orientation layer ofa liquid crystal display element.
 6. Liquid-crystal display element,containing orientation layers containing polyimides according to claim4.
 7. A method of using polyimides according to claim 4 in optical waveguides which comprise incorporating said polyimide into an optical waveguide.
 8. An apparatus containing optical wave guides containingpolyimides according to claim
 4. 9. A process as in claim 2, wherein the--COO(H) groups formed after the tetracarboxylic dianhydride is firstreacted with an aliphatic alcohol or a diamine are converted intoreactive derivatives.
 10. Polyimides which have been prepared frompolyamide esters by alcohol cleavage with cyclization at a curingtemperature of less than 250° C., wherein said polyamide esters:(a) canbe prepared from tetracarboxylic dianhydrides, diamines and alcohols;and (b) have alkoxy radicals OR of the ester groups --COOR, which aresubstituted during the polyamide formation, have 2 to 5 carbon atoms andare monosubstituted or polysubstituted by fluorine.
 11. A liquid crystaldisplay element containing an orientation layer which comprises layerscontaining polyimides according to claim
 10. 12. An optical waveguidewhich comprises polyimides according to claim
 10. 13. Polyimides, as inclaim 4, which have been prepared from polyamide esters by alcoholcleavage with cyclization at a curing temperature of less than 250° C.14. Polyimides, as in claim 4, which have been prepared from polyamideesters of general formula I ##STR16## wherein A is a tetravalent organicradical having at least two C atoms, B is a divalent, organic radicalhaving at least two C atoms, and R is an alkyl radical having 2 to 5 Catoms which is monosubstituted or polysubstituted by fluorine. 15.Polyamide esters containing structural units of a general formula I##STR17## wherein A is a tetravalent organic radical having at least twoC atoms, B is a divalent, organic radical having at least two C atoms,and R is an alkyl radical having 2 to 5 C atoms which is monosubstitutedor polysubstituted by fluorine, which can be prepared fromtetracarboxylic dianhydrides and can be converted into polyimides byalcohol cleavage with cyclization.